Doors are increasingly being manufactured from polymer components. Door facings may be formed from a polymer composite material, such as sheet molding compound (“SMC”). SMC typically includes unsaturated polyester or vinylester resins, thickening agent, and thermoplastic polymers as low shrinkage or low profile additives, a monomer such as styrene, glass fiber reinforcement and inert filler materials like calcium carbonate. Doors having SMC facings resist rot and corrosion, and are generally better insulators than solid wood, wood composite or metal doors. Because of material costs and manufacturing efficiencies, polymer composite door assemblies are less expensive to manufacture than solid wood doors.
A typical compression molding process used in manufacturing SMC door facings involves placing a predetermined weight of SMC, or an SMC charge, within a lower mold half. An upper mold half is then advanced toward the lower mold half to cause the SMC to conform to the shape of the mold. The mold halves are heated to facilitate flow and affect the thermosetting reaction. For producing a “flush” door facing, the upper and lower mold halves are planar. Alternatively, the mold halves may be configured to produce door facings having a contoured profile, for example simulating a paneled door. In addition, the upper mold half may include a textured pattern simulating a wood grain pattern on the exterior surface of the resulting door facing. For example, the upper mold half may include a series of carefully spaced ridges which simulate wood grain ticks, thereby forming a wood grain pattern in the intended visible exterior surface of the door facing. Such wood grain ticks may be formed in the mold half using etching techniques.
Typical door assemblies include a pair of compression molded door facings which are adhesively secured to a rectangular frame that separates and supports the facings in spaced relationship. A cavity is thus created between the facings which may be filled with a core material, such as polyurethane foam. After the facings are secured to the frame, the resulting semi-finished door may be placed between upper and lower heated platens. The foam material is then injected into the cavity through an opening in the frame. The heated platens exert sufficient pressure on the semi-finished door so that injection of the foam does not loosen or ‘blow-out’ the facings from the frame. In addition, the platens may be heated to a temperature sufficient to cause the foam to adhere to the interior surfaces of the facings as the foam expands within the cavity.
The door facings may deform due to the temperature gradient between the heated platens and polymeric material forming the door facings, particularly the upwardly disposed door facing (relative to the lower heated platen). If the door facings are not sufficiently rigid, they tend to sag inwardly toward the cavity due to this temperature gradient during door assembly. In addition, the upwardly disposed facing tends to sag inwardly toward the cavity due to gravitational forces. Thus, the door facings must be sufficiently rigid in order to withstand this inward force, or the facings will sag inwardly toward the cavity, a problem sometimes referred to as ‘dipping’. Such dipping is particularly prevalent when assembling a ‘flush’ door (i.e. a door having planar door facings without contours or a wood grain pattern on their visible surfaces). If extensive dipping occurs during the door assembly process, the resulting door will not be commercially viable.
In order to minimize dipping, the facings must be sufficiently rigid. Some conventional methods of increasing rigidity of molded facings provide for increasing the fiber and/or filler content of the compound used to mold the component. Or, the compound may include an additive such as a cross-linking material, in order to increase the modulus of the compound. The resin system chemistry may also be modified by increasing the cross-linking density of the resulting composite, which also increases the modulus of the compound. Alternatively, the caliper of the door facing may be increased. Yet further, relatively large ribs extending outwardly from the major interior surface of the door facing are provided.
While such methods may improve rigidity of the molded component, the compound formulations are relatively expensive. Further, increasing caliper requires an increase in the amount of material used to form the door facing. The use of ribs also requires an increase in the amount of material used. Furthermore, conventional ribs have a tendency to cause the component being molded to stick to the mold cavity, making it more difficult to remove the component therefrom. Thus, manufacturing costs are increased. In addition, increasing caliper tends to increase cycle time requirements, thereby further increasing manufacturing costs.